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Laminate

Repairing gelcoat cracks and chips

Repairing scratches, crazing and gelcoat chips — opening up the cracks, matching the colour and finishing to a gloss.

A sailing yacht heeling in the wind

Gelcoat cracks for various reasons: impacts, structural stresses, too thick a layer in corners, or ageing of the material. Fine crazing is a network of hairline cracks, while single linear cracks often point to a local impact or overload. Before you repair it cosmetically, establish the cause — if the laminate underneath is damaged or flexible, filling the gelcoat alone will crack again quickly. A purely cosmetic repair only makes sense when the structure is sound.

Author: Maciej Baniewicz · Boat & Yard editorial team

Methodology: The material was prepared from the sources given and from practical service rules; the parameters have to be confirmed in the manual for the specific manufacturer or model.

Scratches and cracks first have to be opened out, that is widened into a V with a small burr or blade, so that the repair gelcoat has something to grip and penetrates the full depth of the crack. Leaving a narrow gap means the new material falls out and the crack comes back. Chamfer the edges of the chip slightly and clean them thoroughly of dirt, grease and release wax with acetone. The surface must be dry and degreased, or the gelcoat will not bond properly.

Matching the colour is often the hardest part of the repair. White gelcoat yellows and dulls with age, so fresh, clean white will stand out — which is why tintable gelcoats are used and samples made on an inconspicuous patch. Repairs use repair gelcoat with hardener added (usually peroxide, MEKP) in exact proportions by weight, because too much or too little spoils the cure. Remember that polyester gelcoat does not cure properly where the surface is in contact with air.

For that reason, repaired gelcoat has to be cut off from oxygen — either with a paraffin additive (wax in solution) or by covering it with PET film or tape after application. Without that the surface stays sticky and cannot be sanded. Apply the material slightly proud of the surrounding surface, because it has to be levelled after curing and sanding. For deeper chips it is better to lay several thinner coats than one thick one, which cracks more easily and cures less well.

Once cured (usually a few to a dozen or more hours, depending on the temperature and the amount of hardener), the film comes off and the repair is wet sanded. Start at 400, then 600, 800, 1000 and up to 1500–2000, levelling the repair with its surroundings and constantly checking that you do not sand through the neighbouring original gelcoat. Then polish the surface with cutting compound and finish with wax, until the repair blends into the rest of the hull. A well-matched colour and careful sanding make the repair almost invisible.

Take care when working with gelcoat and acetone: good ventilation, nitrile gloves, goggles and away from any source of ignition, because the fumes are flammable. MEKP hardener is strongly corrosive — protect your eyes and skin, and keep the bottle away from pigments and accelerators. Work above about 15°C (59°F), because in the cold polyester cures slowly or not at all. Dispose of leftover resin and hardener as the regulations require; never pour them down the drain.

If the cracks are extensive or keep coming back in the same place, it is worth reinforcing the laminate from inside before the repair. Where matching the colour is difficult, consider having the gelcoat matched professionally from the builder's code. On large areas it is sometimes simpler and more durable to repaint the part than to patch it for colour. Patience in tinting and sanding is the difference between a visible repair and a practically invisible one.

Material prices — products and options

Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.

Repair gelcoat (with paraffin or a wax additive)
MEKP hardener, scales / dispenser
Pigments for tinting the colour
Burr / blade for opening out cracks (a V groove)
Acetone / degreaser and cloths
PET film / tape to keep the air out
Wet-and-dry paper 400–2000, polishing compound, wax
Nitrile gloves, goggles, respiratory protection

Common questions

Why does repaired gelcoat stay tacky?

Because polyester gelcoat does not cure properly in contact with air. The repair has to be sealed off from oxygen — with a wax additive or by covering it with PET film or tape right after application. Without that the surface stays sticky and cannot be sanded.

Does a crack have to be opened out before repair?

Yes. The crack is widened into a V with a small burr or blade so the repair gelcoat has something to grip and reaches full depth. Leave it as a narrow slit and the new material falls out and the crack comes back.

Why does fresh white gelcoat stand out against the hull?

Because white gelcoat yellows and dulls with age while the new material is pure white. That is why tintable gelcoats with pigments are used and colour samples are made on a hidden area before repairing a visible one.

One thick layer or several thin ones?

Several thin ones. A thick layer cracks more easily and cures unevenly through its depth. Apply the material slightly proud of the surrounding surface, because once cured it has to be sanded back flush.

When is a cosmetic repair not enough?

When the crack is caused by damaged or flexing laminate underneath. Filling the gelcoat alone will crack again quickly. Do not start surface repairs before establishing the cause, and where structural damage is suspected the boat needs a specialist's inspection.

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